The G protein-coupled receptors (PCRs) comprise a broad family of receptors that activate a large number of effectors in response to a variety of signals including amines, photons, lipids, peptides and proteases. Signaling through CPCRs requires the coordinated balance between proceses that govern receptor activation, desensitization and resensitization. Desensitization of GPCRs involves receptor phosphorylation by specific G protein-coupled receptor kinases (GRKs) and interaction with arrestin proteins. GRKs and arrestins play an important role not only to desensitize second messenger signaling but also contribute to the endocytosis of GPCRs and their ability to recycle back to the plama membrane and resensitize. In addition, the complex of receptor/arrestin engages a variety of signaling pathways including those associated with Src family kinases and components of the MAP kinase cascades. The overall objective of the proposed research is to define the basic molecular and cellular mechanisms that contribute to these processes with the goal of better understanding physiological and pathological conditions. The dopamine and adrenergic receptors will be used as prototypic GPCRs for many of the proposed studies. Aim 1: We will attempt to identify the molecular determinants by which the specificity of action of GRKs and arrestins is established. In addition, we will examine how constitutive phosphorylation and interaction of GPCRs with arrestin can underlie the loss-of-function phenotype of certain mutant GPCRs both in cellular and in vivo systems. Aim 2: The molecular basis of the ability of arrestins to act as endocytic switch and control the resensitization process of GPCRs will be examined. Aim 3: How the pardigm of GRKs and arrestins might apply to non-conventional GPCRS such as frizzled and smoothened will also be examined both in cellular and in vivo model systems. Aim 4: These experiments will explore the hypothesis that the behavioral and biochemical synergisms that exist between dopamine D 1 and D2 receptors results from the oligomerization of these receptors. Results from these studies should broaden our understanding of the role of GPCRs and their regulation in normal physiology and disease states and provide insight into potential new therapeutic approaches for conditions such as heart failure, drug abuse, and psychiatric disorders.